Bone is composed of two kinds of tissue, exterior tissue which is dense in texture (compact tissue) and interior tissue that consists of slender fibers and lamellae that together form a lattice-type structure (cancellous tissue). Damage or loss of bone can result from trauma, congenital anomaly, pathologic conditions (e.g., rheumatoid arthritis, scleroderma, acromegaly and Gauchers disease), and surgical procedures.
In conventional treatment of bone defects, bone-derived or synthetic biomaterials are used to restore form and function. These biomaterials are preferably in the form of porous implant structures having interconnected porous spaces across the substratum of the implant. This allows bone growth into the porous spaces of the implant, securing its incorporation and osteointegration with the surrounding or adjacent viable bone at the margins of the bone defect.
Porous implant structures may be fabricated by a number of manufacturing routes. For implants made according to a standardized format (i.e., not customized for a particular individual) many conventional fabrication techniques can be used, including casting (e.g., ceramic-mold casting, centrifugal casting, die casting, investment casting, lost foam casting, permanent-mold casting, plaster-mold casting, pressure casting, sand casting, shell mold casting, slip casting, squeeze casting, slush casting, vacuum casting), extrusion, laser cutting, machining (e.g., electrochemical machining, water-jet machining), molding (e.g., blow molding, compression molding, injection molding, powder injection molding), thermoforming, and the like.
Implants may also be custom designed using computer-based imaging, processing and modeling techniques to convert common medical images into customized 3D renderings or Computer-Aided Design (CAD) models, which may then be used to fabricate the implant using any number of computer driven manufacturing techniques. The CAD models may be derived from any number of medical diagnostic imaging systems such as computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and x-ray scans. Examples of computer driven manufacturing techniques include fused deposition modeling (FDM), selective laser sintering (SLS), and selective mask sintering (SMS).
Regardless of the manufacturing route, the resulting implant may then be subjected to one or more post-processing steps, which include modifying the implant to include pre-tab holes and other features that aid in rigid affixation.
Examples of synthetic biomaterials used in the fabrication of porous implant structures include ceramics and polymers such as polyethylene, polytetrafluoroethylene (PTFE) and poly(ether ether ketone) (PEEK).
By the late 1990s, PEEK emerged as the leading biomaterial for implants, first being offered commercially as a biomaterial for implants in April 1998. Bolstered by the existence of a stable supply of PEEK in the marketplace, research on PEEK biomaterials has and continues to flourish.
Customized PEEK scaffolds that are fabricated using CAD and rapid prototyping (RP) techniques are described in M. W. Naing et al., FABRICATION OF CUSTOMISED SCAFFOLDS USING COMPUTER-AIDED DESIGN AND RAPID PROTOTYPING TECHNIQUES, Rapid Prototyping Journal, vol. 11, pages 249-259 (2005). In this publication, PEEK-hydroxyapatite (HAP) biocomposite blends are sintered in SLS, with the advantages of HAP reinforced PEEK composites being identified as their strength and stiffness, which are reportedly compatible to that of the bone. PEEK™ 150XF finely ground PEEK powder is used to make these layered scaffolds.
Unfortunately, PEEK processing temperatures are quite high. In addition, less than favorable compressive residual stress profiles have been observed in these customized PEEK scaffolds, attributed to the relatively high solidification rates demonstrated by PEEK materials. Moreover, achieving and maintaining homogeneity in PEEK-HAP powder blends is difficult, with a lack of homogeneity causing the formation of HAP particle clusters in the powder blend. Localized heating of these HAP particle clusters have been found to result in the partial degradation of PEEK and/or the formation of microscale thermal stresses in the resulting scaffold.
By way of the present invention, it has been discovered that poly (ether ketone ketone) or PEKK may be used to make customized implants for bone replacement using rapid prototyping. PEKK offers the benefit of lower solidification rates, and in some embodiments, may also offer the added benefit of considerably lower processing temperatures.
It has also been discovered that customized implants for bone replacement that are prepared from PEKK using rapid prototyping demonstrate biomechanical properties similar (if not identical) to that of natural bone even when prepared without the use of processing aids such as carbon black and aluminum powder. In other words, these implants meet desired shape and strength requirements, which are typically expressed in terms of geometric size and shape, minimum wall thickness and minimum load bearing capacity.
The present invention specifically provides a laser-sinterable PEKK powder product. The laser-sinterable powder is comprised of a PEKK compound resin prepared from semi-crystalline and/or quasi-amorphous PEKK resin, and one or more fillers or additives selected from the group of glass, carbon and mineral fillers. By a “semi-crystalline” or “substantially crystalline” is meant a resin which has at least 10% crystallinity as measured by DSC, preferably from about 15%-90%, and most preferably from about 15-35% crystallinity. By “quasi-amorphous” is meant a resin which has at most 2% crystallinity as measured by DSC. The laser-sinterable powder has an average particle size ranging from about 10 to about 150 microns (preferably, from about 20 to about 100 microns, more preferably, from about 50 to about 70 microns).
The present invention also provides customized implants for bone replacement that are prepared from PEKK using rapid prototyping. The phrase “rapid prototyping”, as used herein, means the automatic construction of physical objects such as implants using sold freeform fabrication.
In a first contemplated embodiment, the customized implant is a rigid implant having an inner core and an outer layer, the inner core having a relatively low porosity of less than about 10%, rendering the implant suitable for replacing bone in load bearing applications such as the spine, long bone and hip. The inventive rigid implant demonstrates a compressive strength (ASTM #D695) or load bearing capability ranging from about 100 to greater than about 200 megapascals (MPa).
Preferably, at least 95% of the pores have a diameter in the range of from about 1 to about 500 microns. Individual pores may or may not be connected to each other.
The implant's outer layer and its inner core preferably match the corresponding regions of the bone to be replaced if that bone were healthy. In other words, the outer layer would approximate the morphologic traits of the compact tissue in the cortical layer of a similar healthy bone, while the inner core would approximate the morphologic traits of the cancellous tissue in the trabecular core of a similar healthy bone.
In a second contemplated embodiment, the customized implant is a less rigid implant with a substantially uniform cross-sectional morphology, which has a higher porosity of greater than about 35%. Such implants are suitable for replacing bone in partially load bearing applications such as scaffolding for ongrowth/ingrowth of tissues, support for stem cell media and the like.
Preferably, individual pores in this less rigid implant are connected to each other, the pores having a diameter in the range of from about 50 to about 250 microns.
The present invention also provides a CAD-based RP process for the design and manufacture of these customized implants, the process comprising:                (a) scanning a patient in an area requiring bone repair or replacement to obtain tomographic information;        (b) designing a bone implant model at a CAD terminal using the tomographic information obtained from the patient;        (c) optionally, modifying the bone implant model by, for example, adding suture anchors, threaded holes, mating surfaces and textures, open cell regions for scaffolding, surface pores to carry antibiotics, and/or varying density or porosity levels so as to vary stiffness or rigidity; and        (d) using a solid free-form fabrication method such as SLS to form a bone implant from the bone implant model, the bone implant comprising sequential layers of biocompatible PEKK.        
In a preferred embodiment, a PEKK compound resin powder is used to form the bone implant, the PEKK powder having a preferred average particle size ranging from about 20 to about 100 microns (more preferably, from about 50 to about 70 microns).
In a more preferred embodiment, a SLS fabrication method is used to form the bone implant, the SLS fabrication method comprising heating a part bed to a temperature ranging from about 280° C. to about 350° C. and scanning a 0.015 to 1.5 Wattsec/millmeter (mm)2 laser spot at selected locations of a layer of PEKK powder contained in the part bed.
Other features and advantages of the invention will be apparent to one of ordinary skill from the following detailed description. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.